Ofcom yesterday fined the BBC for the first time in its 80-year history when it ordered the corporation to pay £50,000 for "serious management failings" that led to Blue Peter viewers being deceived. The BBC had admitted "serious and regrettable errors of judgment" over the incident, which shocked viewers when it emerged at the height of a series of revelations about the misuse of premium phone lines.

When a technical hitch hit the Whose Shoes? phone-in, in which viewers were invited to call at 10p a time to guess the identity of a mystery celebrity in aid of Unicef, a girl on a studio tour was persuaded to pose as the winner.

The incident, which took place last November, came to light three months later when the mother of another child on the tour emailed a Radio 5 Live phone-in show to tell them what she had seen.

The BBC said the mistake had been made by a junior researcher and that as soon as senior executives were aware of it public apologies were issued and an internal review launched.

But Ofcom said the mistake was "set against a background of management and compliance failures" and should not have remained a secret known only to the Blue Peter production team, including the editor, for three months. The BBC also admitted the Blue Peter team did not have the experience to handle such competitions.

Moreover, said the regulator, the editorial needs of the programme overrode the consideration of fairness to the viewer and had a member of the public not complained the problem would almost certainly not have emerged.

The BBC broke rules on conducting competitions fairly and safeguarding the welfare of children and was fined £45,000 for the initial incident and a further £5,000 for allowing the programme to be repeated. During the repeat a further 3,547 callers, mainly children, phoned in to enter a competition that they had no chance of winning.

Ofcom was handed the power to impose cash fines on the BBC for lapses in standards in the last Communications Act after lobbying from commercial rivals.

BBC insiders ridiculed the idea at the time and yesterday said it was "disappointing" that licence fee money should go back to the Treasury over the incident. But the regulator stood by its decision and hinted that if it had been a transgression by a commercial operator the fine would have been even higher.

"Parliament decided that it is appropriate in certain circumstances for Ofcom to fine the BBC," it said. "However, mindful that it is licence payers' money, parliament set a maximum of £250,000 on any one occasion. The committee was aware and took into account the fact that the BBC is publicly funded."

The BBC Trust, created to replace the board of governors this year, said: "The culture of the BBC must be such that any proposal to mislead audiences is instantly dismissed as wholly inappropriate."

The media regulator has already handed out a record £300,000 fine to Five after Endemol-produced lunchtime quiz show Brainteaser was found to have misled viewers on a regular basis by allowing members of the production staff to pose as winners.

Last week premium line regulator Icstis fined interactive services company Eckoh £150,000 for its part in the Richard and Judy You Say We Pay scandal. Ofcom is now investigating Channel 4's role.

The results of an internal BBC investigation, which admitted serious errors on Blue Peter and Saturday Kitchen but claimed a review of more than 200 other programmes had not uncovered any evidence of "systemic abuse or failure", were published in May.

A separate review by former BBC News chief Ron Neil is due to report by the end of the year. Ofcom is investigating more than 20 other allegations of malpractice by broadcasters involving premium phone lines and is also carrying out its own review of the sector.

Executives are readying themselves for a stream of regulatory condemnations that they fear could further damage viewer trust as they battle to maintain loyalty in the face of increased digital competition. ITV chairman Michael Grade recently accused colleagues of displaying a "casual contempt" for viewers.